The present invention relates to the techniques for reading and writing data in a memory medium. It applies in particular to the reading and writing of data in chip cards, these cards being used as travel passes in public transport systems. The reading and writing are typically performed by bus ticketing devices or at metro gates or turnstiles.
The flow of travelers passing through a gate or a turnstile can, in peak periods, be between around 30 people per minute (in Europe) and 60 people per minute (in Asia). Magnetic stripe tickets enable the data to be communicated (read) and updated (written) quickly. In practice, the path of the ticket is the same as that of the traveler.
However, these tickets cannot be used to store a lot of data. An Edmonson format magnetic stripe ticket, for example, contains only 62 useful bits. Furthermore, these tickets cannot be used for secured transactions. Chip cards can be used to store more data. These cards can, furthermore, be equipped with encryption and authentication devices to secure the transactions. These encryption and authentication devices ensure that card writes are limited to those devices that are entitled.
The chip cards can, depending on their type, be read with contact or without contact. The former are read by positioning the chip of the card in contact with a magnetic read head. To this end, these cards are often inserted in a read slot. This mechanical constraint prevents the card from being moved with the traveler to follow the same path. It is therefore not possible to use these cards in the transport domain. Contactless chip cards are therefore used.
The contactless chip cards used in the transport domain normally comply with the standard ISO1443 concerning data interchanges (modulation, carrier). The communication area between card and antenna extends to 10 cm. In order to obtain a fluid motion as the traveler is walking, the duration of a transaction (called the ticket transaction time) must not exceed 150 ms. Above 200 ms, the traveler is subject to a noticeable stoppage time.
If the traveler does not leave his card for long enough in the communication area, the current transaction is interrupted. If the transaction is interrupted in the middle of a write, the stored data is not complete. It is therefore desirable for the write to be performed such that a card is restored to its initial state even when a transaction is interrupted. In other words, the card should contain the initial data if a transaction is interrupted.
Chip cards may or may not be equipped with a microprocessor. Micro-wired chip cards have no microprocessor. The memory space of these cards is divided into blocks. The reader accesses these blocks, in read and write modes, according to its requirements. However, the data on such cards is unstructured: the concept of logical file does not exist. Conversely, those cards equipped with a microprocessor present the stored data in a structured way, in the form of files. Typical of these are the microprocessor cards conforming to the standard ISO 7816-4 or the standard EN 726-3. However, these cards are more expensive than the micro-wired cards.